1. Field of the Invention
Variable pulley transmission assemblies or continuously variable transmissions (CVTs) known in the prior art generally comprise pulleys which (in effect) have an adjustable diameter, a connecting belt and a control unit. Automotive applications for a CVT generally utilize hydrodynamic and/or clutch assemblies as starting devices and to effect a change of direction. A change of belt ratio, when the pulleys are stopped, requires that the belt slide across the pulley faces causing wear on both the belt and the pulley surfaces, and requires a great deal of force to perform such a belt movement. Belt movement force, the rate of ratio change and rate of sheave movement are controlled by the control unit as a function of measured parameters or calculated values.
A significant improvement in variable pulley transmissions is described in U.S. Pat. No. 4,433,594--Smirl entitled "Variable Pulley Transmission", and assigned to the assignee of this application. In that patent, a primary sheave pulley is mounted on an input drive shaft to which a vibration damper is connected; the damper is affixed to a flywheel. A secondary variable sheave pulley is mounted on a second shaft, and connected to the primary pulley by a flexible belt, such that the pulleys are continuously rotating during engine operation. This arrangement provides a CVT mechanism where the pulleys are in continous rotation during engine operation, which rotation provides ease of belt ratio shift even at a stopped or idle position of the engine. Even with the marked improvement achieved by the control arrangement in the described patent, substantial efforts are still being directed to further improve the CVT control systems.
2. The Prior Art
U.S. Pat. No. 4,458,318--Smit et al teaches a control arrangement for a CVT system to govern the sheave diameter of the pulleys and the operation of a slippable starting clutch. The control system includes a conventional computer coupled in series with a control system. Information signals provided to the computer include engine output speed, transmission [or belt] output speed, throttle position, and gearshift position. Utilizing this information, the computer sets desired values for the engine speed, CVT system pressure, and clutch coolant. The control system uses these set signals, with additional information from the CVT fluid system to regulate the CVT ratio and line pressure in the CVT fluid system to maintain the requisite belt tension, and the on-off state of the fluid cooling for the clutch. U.S. Pat. No. 2,647,965--Michie teaches an apparatus utilizing a mechanical linkage to maintain constant tension on a belt in a CVT type transmission by controlling the stroke of the sheaves. This linkage is a simple lever, pivoted in the middle of the free strand of the belt, such that the opening of one sheave is mechanically tied to the closing of the other sheave. Thus a stroke of one sheave is exactly the same as the stroke of the other sheave. This presumes that such opening and closing will maintain constant tension in the belt. However, it can be shown that due to the belt geometry, the sheaves do not open and close at the same rate at constant tension.
Several alternative apparatus for control of CVT-type transmissions utilize a torque sensor associated with one of the shafts and to control the hydraulic pressure in one of the movable sheave chambers to affect the torque of such shaft. In some cases, it is the intent to maintain either an adequate, minimal or constant tension on the belt operable between the sheaves. U.S. Pat. No. 3,600,960--Karig et al illustrates torque sensing control apparatus utilizing a torque-sensing or torque-responsive movement of at least two of the sheaves, which control is stated to be load-responsive to the hydraulic contact pressure applied at the driven side of the transmission. This contact pressure has to be exerted at the driven side to prevent the transmitting element, the belt, from slipping between the two pairs of conical pulley disks. The signal from the torque sensor provides a means of controlling the fluid flow and thereby for maintaining and changing the speed ratio of the transmission. Further, at column 3, line 25, the inherent problem of all the torque-sensing apparatus is noted, that is, that the contact pressure must be greater than desired. A torque sensing control system is also taught in U.S. Pat. No. 3,115,049--Moan, which provides a CVT-type transmission wherein loading of the belt is provided in accordance with the torque transmitting requirements of such transmission.
Torque sensors are utilized for transmission control throughout the automotive industry. Such sensors are provided in U.S. Pat. Nos. 4,450,728--D'Angelo et al and 4,448,275--Kitagawa et al. In the '275--Kitagawa et al patent, a torque sensor generates an electrical signal representative of a change of state of a coupling between a first and second rotor, which rotors are coupled by a torsion spring. This change in state is caused by relative rotation of the rotors, which represents or detects torque in a power assisted steering system of an automotive vehicle. The U.S. Pat. No. 4,450,778--D'Angelo et al patent teaches a vehicle-force measurement system for determining the force output of a vehicle. This system utilizes a fifth wheel to provide an accurate vehicle speed reading, and torque and RPM sensors are used to determine the total power output of the vehicle. The force output of the vehicle is provided as a function of both the total power output and the speed signal.
U.S. Pat. No. 4,292,031--Rattunde teaches a CVT-type transmission utilizing a torque sensor mounted on one of the shafts along with one of the pulleys of such transmission. The torque sensor provides a signal to control the hydraulic pressure in one of the movable sheaves of either of said pulleys. The movable sheave on which the torque sensor is mounted moves against the belt or transmission member with a force which is a function of the torque measured by the torque sensor.
A further example of a control system for a CVT transmission is provided in U.S. Pat. No. 4,246,807--Kofink wherein a torque transducer on the engine drive shaft provides an input to a microprocessor. The microprocessor controls the movable sheaves to determine the transmission ratio of the CVT belt, which ratio is provided by pressing together the halves of one pulley of such belt drive system. In an automotive transmission, there are limits on the belt drive, such that the lowest transmission ratio is automatically produced from the maximum torque regulated speed to the maximum power regulated speed; while at the highest transmission ratio, switching is automatically provided from the engine at its maximum power speed to regulation of the engine at its maximum torque speed. Thus, the belt is operable between a maximum torque and maximum speed at the highest and lowest ratios. However, there is no provision to measure the changes between these limits.
A typical prior art control arrangement is taught in U.S. Pat. No. 3,596,528--Dittrich et al, wherein the fluid pressure on the sheaves is controlled hydraulically. This patent also recognized the desirability of maintaining the contact pressure of the sheaves on the belt so that it is just adequate to prevent slippage of such belt at any speed ratio. A pressure control valve, which is mechanically adjustable by the action of the torque, is used to control the hydraulic pressure to the load responsive as well as the speed responsive portions of the sheaves.
The use of torsional measurements on shafts is well known in the art. Devices and techniques are discussed in an article entitled "Measurement of Torsional Vibration In Rotating Machinery", from the Transactions of the ASME Journal of Mechanisms, Transmissions and Automation in Design, July 1985, wherein several alternative means of torque or torsional measurement on shafts is discussed. Among those methods discussed are the use of strain gages and transducers for providing electromagnetic signals. Problems connected with the use of strain gages and the variations in frequency output, including those differentials for steady state versus dynamic conditions are discussed throughout the article. An optical type transducer is discussed as an alternative means of measurement of torsional response from a shaft. There was no teaching or illustration of the use of strain gages for the measurement of a reaction force in a shaft, especially not related to a CVT transmission.
It has been found that the use of a fixed hydraulic force to control the belt tension, that is, the transmission member, and the torque output during transient changes therein does not provide accurate control in a CVT. Loss of or variations in control leads to low performance shifts during change of ration and can potentially damage the belt, pulley sheaves, or both items. In a CVT, a downshift is generally considered the highest loaded condition, and it is desirable to control the force on the movable sheave to maintain at least a minimal belt tension, as was noted in the prior art cited above. It has been shown and it is known that even at a steady state condition for an automobile, there is between a five and ten percent variation in the belt tension from the theoretically predicted value due to the mechanical variations within the drive train and transmission, that is, broadly speaking, physical deviations from specifications. Maintenance of belt tension during downshifts is an important factor to prevent slip. The present invention provides a means to monitor the tension in the belt throughout the operating range of a CVT transmission. This monitored tension is communicated to a control circuit for control of a CVT.